This invention concerns a device to control the temperature of extruded metallic sections during the extrusion step.
The purpose of the device according to the invention is to monitor the temperature of sections leaving an extrusion press so as to regulate the speed of extrusion according to the temperature of the extruded product, thus enabling an "isothermal extrusion" to be achieved with a constant temperature of the extruded aluminium product by varying the extrusion speed of the press.
The accomplishment of an isothermal extrusion provides advantages as regards shorter extrusion times and better product quality.
So as to understand these facts it is enough to see in FIG. 1 a typical graph of the temperature curve of an average aluminium section produced with a constant extrusion speed of 30 meters per minute.
The graph shows that the highest temperature reached by the extruded aluminium product is 540.degree. C. when starting from a minimum temperature of 410.degree. C.
The value of 540.degree. C. represents the highest temperature which the extruded section should reach since the occurrence of micro surface defects begins above that threshold.
To avoid this occurrence, the speed of the press is regulated in such a way that this temperature is never exceeded.
If on the contrary it is desired to obtain a constant temperature on the extruded product (see FIG. 2), the speed of the press should be varied on the basis of a decreasing speed curve as compared to a constant speed.
A comparison of the two graphs shows that in the latter case the speed of extrusion is optimized, allowing this speed to be increased considerably, above all in the first part of the extrusion process when the temperature of the section is lower.
This enables the average extrusion time to be reduced by a value of the order of 10-20%, depending on the type of the extruded product.
Moreover, extrusion at a constant temperature makes possible a more accurate assessment of the effects of thermal expansion on the cross section of the extruded product precisely because this parameter remains constant along the whole length of the section and therefore enables an exact calculation of the suitable compensation to be carried out.
The above results in a higher product quality.
The present applicant is aware of attempts to govern the extrusion speed by the temperature of the extruded product, but the systems known so far do not enable accurate information to be obtained in view of the reflectance factor of the surface of extruded products, the reflectance being even greater in the case of aluminium or brass.
For this reason the known systems are still generally in the trial stage and do not yield the results hoped for, so that a correct correlation between the extruded temperature and the extrusion speed has still not been achieved industrially.
The contents of document GB-A-547,080 of 1942 are known, wherein a thermocouple is stationarily positioned in a circumferential position on a extruded product near the die. This document discloses monitoring the temperature of the extruded material only at one or more stationary or circumferential positions and depends on a design choice, but it cannot be adapted case by case to the working requirements of the product or of the various products, especially in the case of complex sections such as those of window or door frames.
Document GB-A-1,582,889 of 1978 teaches the use of a pyrometer which is scanned through a pre-determined angle across the path of a body. This system was designed for a rolling cycle and has the purpose substantially of monitoring the temperature of flat surfaces without any complexities of profile. Moreover, this document does not envisage the possibility of monitoring situations of a local temperature nor does it provide for monitoring the temperature in real time and of using this information in real time to govern the instantaneous rolling speed. The teaching of this document cannot be transferred to extrusion presses which extrude complex sections with varied sections such as sections for window or door frames, for instance.